1. Field of the Invention
The invention relates to a circuit arrangement for power generation with solar cells connected in series and in strings of several solar cells, where the strings, to each of which a bypass diode is connected in parallel, are connected in series.
Photovoltaic systems for transforming solar energy into electrical energy comprise solar cells arranged in modules on which are arranged strings of solar cells connected in series and each jumpered by a bypass diode. The strings are also connected in series. The modules with the strings of solar cells are generally of plate-like design. The plates have thin-film solar cells in adjacent strips. The conditions at the installation location of the modules frequently do not allow identical radiation onto all strings of a module, and it may be that strings are in the shade of neighboring objects and hence cannot play any part in power generation.
2. Description of Related Art
To prevent the operation of shaded solar cells, which are for that reason polarized in the non-conducting direction, bypass diodes are used that revert to the conducting state when the strings are in shade. Since the bypass diodes have internal resistances during operation in the conducting direction, heat losses due to current occur in them, thereby reducing the efficiency of the photovoltaic system. The currents flowing via the bypass diodes must not heat up the diodes beyond permitted limit temperatures. Precautions must therefore be taken for heat dissipation by convection and/or radiation, for example by mounting the bypass diodes on heat sinks.
The strings are provided with connections for external lines. The lines from and to the strings are arranged with their one end in connection boxes in which the strings are connected to one another.
When numerous strings and possibly solar modules are connected in series, relatively high currents flow via the bypass diodes of individual strings when the latter are in shade. The number of strings in shade corresponds to the number of conducting bypass diodes. The energy yield of a photovoltaic system is therefore also reduced by the bypass diodes in an unwelcome way when the strings are in shade.
A solar generator is described in DE 88 15 963.9U1, in which each solar cell module is connected in parallel to a shunt diode and the latter to an LED, in order to rapidly detect a defective solar cell module by visual means.